Control device for motorized power steering device

ABSTRACT

The present invention provides an electric power steering apparatus for controlling a motor based on a current command value calculated from a steering assist command value calculated based on a steering torque generated in a steering shaft and from a current detection value of the motor which gives a steering mechanism a steering assist force. In order to realize ideal road information sensitivity and steering feeling in frequency region at the same time, the electric power steering apparatus comprises a torque filter for processing a torque signal, an SAT estimating function, and an SAT filter for signal-processing SAT information from the SAT estimating function, and the electric power steering apparatus also comprises a control system with two degree of freedom which is capable of independently designing frequency characteristics of steering feeling and road information sensitivity.

TECHNICAL FIELD

The present invention relates to an electric power steering apparatus inwhich a steering assist force caused by a motor is given to a steeringsystem of a car or a vehicle, and more particularly, to an electricpower steering apparatus comprising a control system with two degree offreedom which is capable of independently designing steering feeling androad information sensitivity to realize ideal steering feeling and roadinformation sensitivity at the same time in a frequency region.

BACKGROUND TECHNIQUE

The electric power steering apparatus for assist-load energizing thesteering apparatus of a car or a vehicle with rotation force of a motoris designed to assist-load energize a steering shaft or a rack shaftwith a driving force of the motor by a transmission mechanism such as agear or a belt through a speed reducer. In such a conventional electricpower steering apparatus, feedback control of motor current is performedto precisely generate assist torque (steering assist torque). Thefeedback control is for adjusting voltage to be applied to the motorsuch as to reduce a difference between a current command value and amotor current detection value, and the voltage to be applied to themotor is generally conducted by adjusting a duty ratio of a PWM (pulsewidth modulation) control.

A general structure of the electric power steering apparatus will beexplained with reference to FIG. 7. A column shaft 2 of a steering wheel1 is connected to a tie rod 6 of a running wheel through a speedreducing gear 3, universal joints 4 a and 4 b and a pinion rackmechanism 5. The column shaft 2 is provided with a torque sensor 10 fordetecting the steering torque of the steering wheel 1. A motor 20 forassisting the steering torque of the steering wheel 1 is connected tothe column shaft 2 through the speed reducing gear 3. Electricity issupplied to a control unit 30 which controls the power steeringapparatus from the battery 14 through an ignition key 11 and a relay 13.The control unit 30 calculates a steering assist command value I ofassist command based on the steering torque T detected by the torquesensor 10 and based on vehicle speed V detected by the vehicle speedsensor 12, and the control unit 30 controls current to be supplied tothe motor 20 based on the calculated steering assist command value I.

The control unit 30 mainly comprises a CPU (including MPU also), andgeneral functions of the control unit 30 executed by program in the CPUare shown in FIG. 8. For example, a phase compensator 31 is not a phasecompensator as independent hardware, but is a phase compensatingfunction executed by the CPU.

The functions and operation of the control unit 30 will be explained.The steering torque T which is detected and inputted by the torquesensor 10 is compensated in phase by the phase compensator 31 forenhancing the safety of the steering system. The steering torque TAwhich was compensated in phase is input to a steering assist commandvalue calculating element 32. The vehicle speed V detected by thevehicle speed sensor 12 is also input to the steering assist commandvalue calculating element 32. The steering assist command valuecalculating element 32 determines the steering assist command value Iwhich is a control target value of current to be supplied to the motor20 based on the input steering torque TA and vehicle speed V. Thesteering assist command value I is input to a subtracter 30A and is alsoinput to a differentiation compensator 34 of a feedforward system forenhancing the response speed. A deviation (I-i) of the subtracter 30A isinput to a proportion calculating element 35 and is also input to anintegration calculating element 36 for improving the characteristics ofthe feedback system. The output of the differentiation compensator 34and outputs of the proportion calculating element 35 and the integrationcalculating element 36 are added and input to an adder 30B. A currentcontrol value E which is a result of addition in the adder 30B is inputto a motor drive circuit 37 as a motor driving signal. The current i ofthe motor 20 is detected by a motor current detection circuit 38 and isfed back to the subtracter 30A.

In such a conventional electric power steering apparatus, in order tomake full use of the flexibility in design thereof and to obtain stableand comfortable steering feeling, it is proposed to design thetransmitting characteristics of the steering feeling and the roadinformation sensitivity in the frequency region (e.g., Japanese PatentApplication Laid-open No. 2001-334948). That is, the complementarysensitivity function with respect to frequency of the control apparatusis set a value approaching “1” in a band where there exists disturbanceto be suppressed, and is set to a value approaching zero in a band wherethere exists disturbance to be transmitted. This is because that underthe definition of the complementary sensitivity function, when thefunction is “1”, this means that the disturbance is suppressed, and whenthe function is zero, this means that the disturbance is not suppressedat all and is transmitted.

It is conceived that the steering feeling and the road informationsensitivity can be used as indices of performance evaluation in theelectric power steering apparatus. However, the conventional electricpower steering apparatus has a problem that since the transmittingcharacteristics representing these two indices have mutual subordinativerelation, it is difficult to design a control system which satisfies thetwo indices at the same time. That is, since the steering feeling andthe road information sensitivity have the mutual subordinative relation,the steering feeling and the road information sensitivity can not bedesigned independently, and it is difficult to design a controlapparatus which satisfies the ideal steering feeling and roadinformation sensitivity at the same time.

The present invention has been accomplished in view of the abovecircumferences, and it is an object of the invention to provide anelectric power steering apparatus comprising a control system with twodegree of freedom which is capable of independently designing thesteering feeling and road information sensitivity for realizing idealsteering feeling and road information sensitivity at the same time in afrequency region.

DISCLOSURE OF THE INVENTION

The present invention relates to an electric power steering apparatusfor controlling a motor based on a current command value calculated froma steering assist command value calculated based on a steering torquegenerated in a steering shaft and from a current detection value of themotor which gives a steering mechanism a steering assist force. Theabove object of the invention is achieved by the following structure.That is, the electric power steering apparatus comprises a torque filterfor processing a torque signal, an SAT estimating function, and an SATfilter for signal-processing SAT information from the SAT estimatingfunction, and wherein the electric power steering apparatus alsocomprises a control system with two degree of freedom which is capableof independently designing steering frequency characteristics ofsteering feeling and road information sensitivity.

Further, the above object of the invention is more effectively achievedby the following structure. That is, gain of the steering feeling is setsuch that the gain maintains at a constant value up to frequency as highas possible, or the road information sensitivity can eliminateinformation in an unnecessary frequency band, or the unnecessaryfrequency band is set in a range of 10 Hz to 30 Hz.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure example of an embodimentof the present invention,

FIG. 2 is a block diagram showing a structure example of a control unit,

FIG. 3 is a schematic diagram of FIG. 2,

FIG. 4 is a diagram used for explaining estimation of SAT,

FIG. 5 is a diagram showing target transmission characteristics ofsteering feeling,

FIG. 6 is a diagram showing target transmission characteristics of roadsensitivity information,

FIG. 7 is a diagram showing a structure example of a general powersteering apparatus, and

FIG. 8 is a block diagram showing a structure example of a control unit.

BEST MODE FOR CARRYING OUT THE INVENTION

An electric power steering apparatus of the present invention controls amotor based on a current command value calculated from a steering assistcommand value calculated on the basis of a steering torque generated ina steering shaft and from a current detection value of the motor whichgives steering assist force to a steering mechanism. The electric powersteering apparatus comprises a torque filter for processing a torquesignal from the torque sensor, an SAT estimating function whichestimates a self aligning torque (SAT) based on a torque signal, a motorangular speed and the like, and a SAT filter for processing a signal ofSAT information estimated by the SAT estimating function. The electricpower steering apparatus has a control system with two degree of freedomwhich is capable of independently designing steering feeling(transmission characteristics from the steering angle of a steeringwheel to the steering torque), and road information sensitivity(transmitting characteristics from road surface reaction to steeringtorque). Thus, the invention has a merit that the control system capableof satisfying the ideal steering feeling and road informationsensitivity characteristics at the same time can easily be designed.

The road information sensitivity may be adjusted by changing thesuspension characteristics of a car.

An embodiment of the present invention will be explained in detail withreference to the drawings. The invention can be applied to all electricpower steering apparatuses irrespective of electric power steering types(column type, pinion type, rack type and the like) and irrespective ofkinds of motor (having brush or brushless).

The present invention provides the control system with two degree offreedom which is capable of independently adjusting and designingfrequency characteristics of the steering feeling and road informationsensitivity, and the control system with two degree of freedom comprisesthe torque filter with respect to the torque signal, the SAT estimatingfunction for estimating and outputting the SAT, and the SAT filtercapable of signal-processing the SAT estimated value estimated by theSAT estimating function in a frequency region. As a result, it ispossible to easily design a control system capable of satisfying idealsteering feeling and road information sensitivity characteristics at thesame time.

FIG. 1 is a block diagram of the entire structure of the electric powersteering apparatus of the invention. A steering torque Th from thesteering wheel is transmitted to a running wheel (transmitting function:1/J_(h)s²) 51 through a subtracter 50, and is input to a controller C(s)through another subtracter 52. A block 53 in the controller C(s)represents rigidity (transmitting function: K) of a torsion bar. Outputof the controller C(s) is input to a steering model (transmittingfunction: P(s)) 60 through a subtracter 64, output θ_(g) of the steeringmodel 60 is fed back to the subtracter 52, and is output through anoverall steering gear ratio (transmitting function: α) 61, and is inputto the subtracter 64 as SAT information through a vehicle model(transmitting function: C(sI-A)⁻¹B+D) 62 and a feedback block(transmitting function: 1/α) 63. The transmitting functions of thesteering model 60 and the vehicle model 62 are known.

The controller C(s) comprises a control unit 100, a speed reduction gearratio (transmitting function: r_(g1)/r_(g2)) 55, a torsion bar(transmitting function: K) 53 and an adder 54. The controller C(s)drives and controls a steering assist motor 200. A motor current im ofthe motor 200 is input to the control unit 100, and is input to an adder54 through a torque constant (transmitting function: K_(t)) 201 of themotor and the speed reduction gear ratio 55. FIG. 2 is a detailed blockdiagram of the control unit 100. The control unit 100 comprises a torquecontroller 110 and a motor drive system 140, and drives and controls themotor 200 through a motor drive unit 202.

A torque signal Tr is input to an assist amount calculating unit 111, adifferentiation controller 112, a yaw rate convergence controller 122and an SAT estimating function 120. A vehicle speed signal Ve1 is inputto the assist amount calculating unit 111 and the yaw rate convergencecontroller 122. The output of the assist amount calculating unit 111 isoutput to the differentiation controller 112 and is input to the adder113, a result of the addition is input to the torque filter 114 and issignal-processed, and the signal-processed filter output is input to theSAT estimating function 120 and is input to a robust stabilizingcompensator 116 through an adder 115. The output of the yaw rateconvergence controller 122 is input to the adder 115. The SATinformation from the SAT estimating function 120 is signal-processed inan SAT filter 121, and is input to the subtracter 117 together withoutput of the robust stabilizing compensator 116 and is subjected tosubtraction processing.

The output of the robust stabilizing compensator 116 is input to anadder 141 in the motor drive system 140 through the subtracter 117, andits addition result is input to the adder 143 through a compensator 142,its addition result is input to the motor drive unit 202 and is input toa disturbance estimator 144. The motor 200 is driven by output (terminalvoltage) Vm of the motor drive unit 202, the output Vm and the motoroutput current im are input to a motor angular speed estimator 145, andthe motor output current im is also input to a disturbance estimator144. A motor angular speed ω estimated by the motor angular speedestimator 145 is input to the motor angular acceleration estimator 146,the yaw rate convergence controller 122 and the SAT estimating function120. A motor angular acceleration *ω from the motor angular accelerationestimator 146 is input to a motor inertia compensator 147 and to the SATestimating function 120.

In such a structure, the assist amount calculating unit 111 calculatesthe assist amount in accordance with a predetermined calculationequation based on the torque signal Tr and the vehicle speed signal Ve1,the differentiation controller 112 enhances the control response in thevicinity of a neutral point of the steering operation, and realizessmooth steering operation. The robust stabilizing compensator 116 is acompensator disclosed in Japanese Patent Application Laid-open No.H8-290778 for example. The robust stabilizing compensator 116 has acharacteristic equation G(s)=(s²+a1·s+a2)/(s²+b1·s+b2) wherein s isLaplace operator. The robust stabilizing compensator 116 eliminates apeak value of resonance frequency of a resonance system comprisinginertia element and spring element included in the torque signal Tr, andcompensates a deviation in phase of the resonance frequency whichhinders stability and response of the control system. The yaw rateconvergence controller 122 applies the brakes to shaky motion of asteering wheel to improve the convergence of the yaw of a vehicle. Themotor inertia compensator 147 multiplies the motor angular acceleration*ω by gain (several steps) to obtain a motor inertial compensationvalue. The disturbance estimator 144 is an apparatus as disclosed inJapanese Patent Application Laid-open No. H8-310417 for example. Thedisturbance estimator 144 can maintain a desired motor controlcharacteristics in an output reference of the control system based onthe motor output current im and a signal which is obtained by addingoutput of the disturbance estimator 144 to a current command valuecompensated by the compensator 142 which is a control target of themotor output. The disturbance estimator 144 prevents the stability ofthe control system from being lost. The angular speed is estimated inthe motor angular speed estimator 145 using a known method based on themotor terminal voltage Vm and the motor output current im.

Here, the SAT estimating function 120 will be explained. Japanese PatentApplication No. 2001-171844 filed by the present assignee disclosesfunction. This function can be applied to the SAT estimating function120. The outline of this application will be explained.

FIG. 4 shows torque generated from a road surface to the steering. If adriver steers the steering wheel, a steering torque Th is generated, andmotor generates assist torque Tm in accordance with the steering torqueTh. As a result, running wheels are steered, and SAT is generated asreaction force. At that time, inertia J of the motor and friction(static friction) Fr generate resistance against the steering operation.If the balance of these forces is conceived, the following kineticequation (1) is obtained:J·*ω+Fr·sign(ω)+SAT=Tm+Th   (1)

Here, the equation (1) is Laplace converted while using zero as aninitial value, and if SAT is solved, the following equation (2) isobtained:SAT(s)=Tm(s)+Th(s)−J·*ω(s)+Fr·sign(ω(s))   (2)

As is found from the equation (2), if the inertia J of the motor and thestatic friction Fr are previously obtained as constants, SAT can beestimated from the motor angular speed ω, the motor angular acceleration*ω, the steering assist force and the steering signal. From this reason,outputs of the torque signal Tr, the motor angular speed ω, the motorangular acceleration *ω and torque filter 114 of the assist amountcalculating unit 111 are input to the SAT estimating function 120.

If the SAT information estimated by the SAT estimating function 120 isfed back as it is, the steering force becomes excessively large, and thesteering feeling can not be enhanced. Thus, the estimated SATinformation is signal-processed using the SAT filter 121, and onlyinformation that is necessary for enhancing the steering feeling isoutput.

Next, the structure of the control system with two degree of freedomwill be explained. The control unit 100 shown in the block diagram ofFIG. 1 has the structure as shown in FIG. 2. The control unit 100comprises the torque filter 114 for processing the torque signal Tr, theSAT estimating function 120 for estimating the SAT, and the SAT filter121 for signal-processing the SAT information from the SAT estimatingfunction 120. In the block diagram of FIG. 1, the rigidity K of thetorsion bar 53 and the controller of the electric power steering arecombined as a new controller C(s), and the steering system is defined asP(s), and a general control system is as shown in FIG. 3. In FIG. 3, ifthe operation amount u which is output from the controller C(s) isconceived, a relation of the following equation (3) is obtained:u=Ts+Tm−Q·SÂT   (3)

In the equation (3), Ts represents steering torque (detection value), Tmrepresents assist torque, Q represents the SAT filter 121 capable ofprocessing the SAT estimated value in the frequency region, and hat SATrepresents an estimated value estimated by the SAT estimating function.Here, if a controller except the SAT estimating function 120 and the SATfilter 121 is defined as C′ (s) and the estimated SAT and the actual SATare equal to each other (SAT=hat SAT), the following transmittingfunction equation is obtained. A torque filter 114 with respect to thetorque signal Tr is also included in the controller C′ (s).$\begin{matrix}{\theta_{g} = {{\frac{PC}{1 + {PC}^{\prime}}\theta_{h}} - {\frac{\left( {1 + Q} \right)P}{1 + {PC}^{\prime}}T_{sat}}}} & (4)\end{matrix}$

The steering feeling (transmitting characteristics from the steeringangle θ_(h) of the steering wheel to steering torque Ts) Gsf and roadinformation sensitivity (transmitting characteristics from the roadreaction force SAT to the steering torque Ts) Gds are conceived asevaluation function in the control apparatus of the electric powersteering apparatus. Therefore, if Ts=K(θ_(h)−θ_(g)) indicative of thedetection characteristics of torque is substituted into the equation(3), the following equation (5) is obtained: $\begin{matrix}{T_{s} = {\frac{K}{1 + {{PC}^{\prime}\theta_{h}}} - {\frac{K\left( {1 + Q} \right)P}{1 + {PC}^{\prime}}T_{sat}}}} & (5)\end{matrix}$

Here, the road information sensitivity Gds and the steering feeling Gsfcan be expressed as the following equations (6) and (7):Gds={K(1+Q)P}/(1+PC′)   (6)Gsf=K/(1+PC′)   (7)

From the equations (6) and (7), it can be found that there exists arelation of the following equation (8) between the road informationsensitivity Gds and the steering feeling Gsf:Gds=Gsf·P(1+Q)   (8)

Here, K represents the rigidity of the torsion bar, and P represents asubject to be controlled. Therefore, the control system is designed suchthat the controller C′ and the SAT filter Q are adjusted so that theroad information sensitivity Gds and the steering feeling Gsf havepreferable characteristics. To design the control system, the controllerC′ is first adjusted so that the steering feeling Gsf has preferablecharacteristics and then, the SAT filter Q is adjusted so that the roadinformation sensitivity Gds has the preferable characteristics. However,when the system does not have the SAT estimating function 120 and theSAT filter 121 (Q=0), since the control system with two degree offreedom can not be constituted, it is necessary to provide the roadinformation sensitivity Gds and the steering feeling Gsf with thepreferable transmitting function by adjusting the controller C′ only.For this reason, it is difficult to design a control system whichsatisfies the road information sensitivity Gds and the steering feelingGsf at the same time.

Thus, as in the present invention, the controller C′ (s) including thetorque filter 114 with respect to the torque signal Tr is first tuned sothat the transmitting characteristics of the steering feeling Gsf hasthe preferable characteristics and then, the SAT filter 121 (Q) is tunedso that the transmitting characteristics of the road informationsensitivity Gds has the preferable characteristics. With this, itbecomes easy to design a control system which satisfies the twoevaluation functions at the same time. That is, the control system withtwo degree of freedom can be constituted.

FIG. 5 shows target transmitting characteristics of the steeringfeeling. Here, a gain Ksf increases or reduces the weight of thesteering operation. If the gain Ksf is greater, the steering operationbecomes heavier, and if the gain Ksf is smaller, the steering operationbecomes lighter. The characteristics are determined by a driver'stastes. In view of steering following ability, it is preferable todesign such that the gain becomes constant up to frequency as high aspossible (fst1 is above 5 Hz and fst2 is depends on characteristics ofvehicle and motor).

FIG. 6 shows the target transmitting characteristics of road sensitivityinformation. The road information is one of important information for adriver. However, since a driver knows that unnecessary road informationexists between 10 Hz and 30 Hz based on his or her experience,preferable characteristics are such frequency characteristics that roadinformation in the frequency region is not transmitted to the steeringoperation. Therefore, the control system is designed such that the lowerlimit frequency fda1 is about 10 Hz, and the upper limit frequency fda2is about 30 Hz.

The roles of the torque filter 114 and the SAT filter 121 are conceivedas adjusting parameters for designing the control system, and it isconceived that the characteristics required for the torque filter 114and the SAT filter 121 are varied depending upon vehicle'scharacteristics and other controller's characteristics. Therefore, thetarget following ability characteristics shown in FIG. 5 andcharacteristics required for realizing the road surface sensitivitycharacteristics shown in FIG. 6 are the characteristics required for thetorque filter 114 and the SAT filter 121. That is, the torque filter 114with respect to the torque signal Tris used for improving the followingability characteristics of the electric power steering apparatus, and ifthe characteristics of the torque filter 114 with respect to the torquesignal Tr included in C′ in the equation (5) is adjusted, the followingability characteristics shown in FIG. 5 are realized. The SAT filter 121is used for adjusting the road information sensitivity characteristics,and if the characteristics of the SAT filter 121 is adjusted, the roadsurface sensitivity characteristics are realized.

INDUSTRIAL APPLICABILITY

According to the present invention, the control system with two degreeof freedom comprises a torque filter for processing a torque signal, anSAT estimating function, and an SAT filter capable of processing theestimated SAT information in a frequency region. Thus, the steeringfeeling and the road information sensitivity can independently bedesigned. As a result, it is possible to easily design a control systemcapable of satisfying the ideal road information sensitivity andsteering feeling at the same time.

1. An electric power steering apparatus for controlling a motor based ona current command value calculated from a steering assist command valuecalculated based on a steering torque generated in a steering shaft andfrom a current detection value of the motor which gives a steeringmechanism a steering assist force, comprising: a torque filter forprocessing a torque signal; an SAT estimating function; and an SATfilter for signal-processing SAT information from said SAT estimatingfunction, wherein: said electric power steering apparatus also comprisesa control system with two degree of freedom which is capable ofindependently designing frequency characteristics of steering feelingand road information sensitivity.
 2. An electric power steeringapparatus according to claim 1, wherein gain of said steering feeling isset such that the gain maintains at a constant value up to frequency ashigh as possible.
 3. An electric power steering apparatus according toclaim 1, wherein said road information sensitivity can eliminateinformation in an unnecessary frequency band.
 4. An electric powersteering apparatus according to claim 3, wherein said unnecessaryfrequency band is in a range of 10 Hz 30 Hz.
 5. An automobile which isequipped with the electric power steering apparatus according to any oneof the preceding claims 1 to 4.